The present invention relates to blades for a steam turbine rotor. More specifically, the present invention relates to a high performance controlled reaction blade for use in the stage that is one stage upstream from the next to the last stage in a low pressure steam turbine.
The steam flow path of a steam turbine is formed by a stationary cylinder and a rotor. A large number of stationary vanes are attached to the cylinder in a circumferential array and extend inward into the steam flow path. Similarly, a large number of rotating blades are attached to the rotor in a circumferential array and extend outward into the steam flow path. The stationary vanes and rotating blades are arranged in alternating rows so that a row of vanes and the immediately downstream row of blades forms a stage. The vanes serve to direct the flow of steam so that it enters the downstream row of blades at the correct angle. The blade airfoils extract energy from the steam, thereby developing the power necessary to drive the rotor and the load attached to it.
The amount of energy extracted by each row of rotating blades depends on the size and shape of the blade airfoils, as well as the quantity of blades in the row. Thus, the shapes of the blade airfoils are an extremely important factor in the thermodynamic performance of the turbine and determining the geometry of the blade airfoils is a vital portion of the turbine design.
As the steam flows through the turbine its pressure drops through each succeeding stage until the desired discharge pressure is achieved. Thus, the steam properties--that is, temperature, pressure, velocity and moisture content--vary from row to row as the steam expands through the flow path. Consequently, each blade row employs blades having an airfoil shape that is optimized for the steam conditions associated with that row. However, within a given row the blade airfoil shapes are identical, except in certain turbines in which the airfoil shapes are varied among the blades within the row in order to vary the resonant frequencies.
The difficulty associated with designing a steam turbine blade is exacerbated by the fact that the airfoil shape determines, in large part, the mechanical strength of the blade and its resonant frequencies, as well as the thermodynamic performance of the blade. These considerations impose constraints on the choice of blade airfoil shape so that, of necessity, the optimum blade airfoil shape for a given row is a matter of compromise between its mechanical and aerodynamic properties.
Generally, major losses in the blade row may occur due to four phenomena--(i) friction losses as the steam flows over the airfoil surface, (ii) losses due to separation of the boundary layer on the suction surface of the blade, (iii) secondary flows in the steam flowing through the channel formed by adjacent blades and the end walls, and (iv) steam leakage past the blade tip. Friction losses are minimized by maintaining the velocity of the steam at relatively low values. Separation of the boundary layer is prevented by ensuring that the steam does not decelerate too rapidly as it expands toward the trailing edge of the airfoil. Losses due to secondary flow and tip leakage may be minimized by controlling the radial reaction distribution along the airfoil.
In a reaction turbine, the airfoils of the stationary vanes and the rotating blades are designed so that a portion of the stage pressure drop occurs in the row of vanes and essentially the balance of the stage pressure drop occurs in the row of blades. The degree of reaction in a turbine stage is defined as the percentage of the stage pressure drop that occurs in the rotating blade row and is an important parameter in blade design. Traditionally, the reaction at the base of the blade airfoil was maintained at approximately 10-15%--that is, in the vicinity of the hub of the stage, 10-15% of the stage pressure drop occurred in the row of blades and 85-90% occurred in the upstream row of vanes. The reaction at the tip of the airfoil was traditionally maintained at approximately 65%. However, such a radial reaction distribution can result in significant secondary flow at the base of the airfoil and high leakage across the tip of the airfoil, both of which adversely affect the performance of the blade, as explained above.
It is therefore desirable to provide a row of steam turbine blades that provides high performance by use of an airfoil shape that maintains the steam velocity at relatively low values, ensures that the steam does not decelerate too rapidly as it expands toward the trailing edge, and controls the reaction so as to produce a radial reaction distribution that tends to minimize secondary flow at the base of the airfoil and steam leakage at the tip.